Materials Included In Kit

Additional Materials Required

Prelab Preparation

“Known” Sample for Qualitative Analysis: Mix 33 mL of the three metal nitrate stock solutions (see above) in a 250-mL beaker to prepare 100 mL of a known sample containing 0.05 M of each metal ion.

“Unknown” Samples for Qualitative Analysis: See Table 1. Prepare about 30 mL each of six different unknowns (three individual metal nitrates plus three different possible combinations of two metal nitrates (e.g., iron(III) nitrate/silver nitrate, iron(III) nitrate/zinc nitrate and silver nitrate/zinc nitrate). Add distilled water as needed to give a final concentration of 0.05 M of each metal nitrate in each unknown. Randomly distribute the six possible unknowns among 30 labeled vials to give each student group a unique unknown.

Safety Precautions

Sodium hydroxide and ammonia solution are corrosive liquids and especially dangerous to the eyes; skin burns are possible. Ammonia is also toxic by inhalation. Hydrochloric acid is toxic by ingestion and corrosive to skin and eyes. Silver nitrate solution is a skin and eye irritant and is slightly toxic by ingestion; it will stain skin and clothing. Iron(III) nitrate solution may also be irritating to body tissues. Potassium ferrocyanide and potassium thiocyanate solutions are slightly toxic by ingestion and may liberate toxic hydrogen cyanide gas upon contact with concentrated acids. Avoid contact of all chemicals with eyes and skin. Keep sodium carbonate and citric acid on hand to clean up acid and base spills, respectively. Wear chemical splash goggles and chemical-resistant gloves and apron. Please review current Safety Data Sheets for additional safety, handling and disposal information. Remind students to wash their hands thoroughly with soap and water before leaving the lab.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Excess sodium hydroxide and ammonium hydroxide may be neutralized with acid and disposed of according to Flinn Suggested Disposal Method #10. Excess hydrochloric acid may be neutralized with base and disposed of according to Flinn Suggested Disposal Method #24b. The reaction mixtures may be washed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• For best results, schedule at least two 50-minute laboratory periods for completion of this assignment. The qualitative analysis scheme, with positive identification of each cation, can probably be completed in one 50-minute lab period. Scheduling two lab periods for this activity, however, will allow students time to “get their hands wet” as they gain practical experience of how much precipitating agent to add or what a color test will look like. The time may also be used to carry out reference tests with known reference samples, as described in the Procedure section.
• Student preparation is the most important element for success in a student-directed, inquiry-based activity. The Prelab Questions may be assigned to help students plan their tests and to lead a class discussion before students begin work in the lab. To ensure a safe lab environment, it is essential that the teacher check students’ procedures and their understanding of the necessary Safety Precautions, as recommended in the Procedure.
• A detailed procedure, complete with sample data, is included in these teacher’s notes for convenience. The procedure may be used as an alternative student handout, if desired.

Teacher Tips

• Typically in a qualitative analysis experiment the objective is to analyze an unknown and get the “right” answer. Getting the right answer is always good, of course, but it should not obscure the broader purposes of this lab. Qualitative analysis, done right, allows students to develop critical-thinking and logical-reasoning skills. It also helps them apply what they have learned in one context (chemical reactions) for another purpose, in this case, chemical analysis.
• See the An Example of Qualitative Analysis in the Discussion section for an explanation of the principles and design of qualitative analysis. The example illustrates the reactions involved in the separation and identification of copper, lead and zinc ions and includes a flow chart diagram. This example may be used as additional background material to help students analyze the reactions of iron(III), silver and zinc ions in this experiment.

Answers to Prelab Questions

1. What precipitation reaction could be used to separate and remove silver ions from a mixture containing iron(III), silver and zinc ions?

Silver ions can be selectively precipitated from a mixture containing iron(III), silver and zinc ions using hydrochloric acid. Silver chloride (AgCl) will precipitate out as an insoluble white solid and can be separated from the remaining aqueous solution containing soluble Fe³⁺ and Zn²⁺ ions.

2. What precipitation reaction could be used to separate and remove iron(III) ions from the remaining mixture containing iron(III) and zinc ions?

Iron(III) ions can be selectively precipitated from a mixture containing both iron and zinc ions using an excess of sodium hydroxide solution. Iron(III) hydroxide [Fe(OH)₃] will precipitate out as an insoluble red solid and can be separated from the remaining solution of soluble Zn(OH)₄^2– complex ions. Note: Stress the importance of adding an excess of sodium hydroxide to prevent the precipitation of Zn(OH)₂, which is insoluble in water.

3. How can the presence of silver ions be positively identified?

Silver ions can be positively identified by redissolving the initial silver chloride precipitate using excess ammonia. The resulting complex ion product [Ag(NH₃)²⁺] is soluble in water.

4. How can the presence of iron(III) ions be positively identified?

The presence of iron(III) ions can be positively identified by redissolving the Fe(OH)₃ precipitate in excess hydrochloric acid and adding potassium thiocyanate, which converts soluble Fe³⁺ ions to characteristic red complex ions having the formula FeSCN²⁺.

5. How can the presence of zinc ions be positively identified?

Zinc ions can be positively identified by acidifying the soluble Zn(OH)₄^2– ions remaining in solution and then precipitatingthe Zn2+ ions with potassium ferrocyanide.

6. Complete the following flow chart. Write the formulas of the reactants next to each added step and enter the formulas of the products in the boxes.

Sample Data

Sample Procedure

Part A. Separation of Metal Ions

1. Label five test tubes 1–5. Place about 1 mL (20 drops) of the solution to be analyzed into test tube 1. Record the sample identity and the color of the solution in the data table.
2. Add 5 drops of 3 M HCl and stir to mix. Record the appearance and color of any precipitate.
3. Centrifuge the mixture to separate the solid, if necessary.
4. Test to be sure precipitation is complete: Add one more drop of 3 M HCl to the supernatant. If more precipitate appears, continue adding 3 M HCl dropwise until no more solid forms.
5. Centrifuge and decant (pour off) the supernatant into test tube 2. Alternatively, use a Beral-type pipet to remove the supernatant. Record the color and appearance of the solution in the data table.
6. Save the precipitate in test tube 1 for Part B, step 11.
7. Add 5 drops of 6 M NaOH to the solution in test tube 2.
8. Stir the solution and test with litmus paper to be sure the solution is basic, then add 3 more drops of 6 M NaOH. Record the appearance and color of any precipitate.
9. Centrifuge the mixture to separate the solid, if necessary. Decant the supernatant into test tube 3. Alternatively, use a Beral-type pipet to remove the supernatant. Save the precipitate in test tube 2 for use in Part B, step 13.
10. Record the color and appearance of the solution in test tube 3 and save the solution for use in Part B, step 16.

11. Rinse the precipitate in test tube 1 with 10 drops of distilled water. Tap or swirl the test tube to mix the contents. Centrifuge the mixture and decant the rinse water.
12. Add 8–10 drops of 6 M NH₃ to the solid in test tube 1 and stir to mix. Record observations in the data table.
13. Rinse the precipitate in test tube 2 with 10 drops of distilled water. Tap or swirl the test tube to mix the contents. Centrifuge the mixture and decant the rinse water.
14. Add 5–7 drops of 3 M HCl to the solid in test tube 2 and stir to mix. Test the solution with litmus paper to be sure it is acidic. Record observations in the data table.
15. Add 3 drops of 0.2 M KSCN to the solution in test tube 2 and record observations in the data table.
16. Add 8–10 drops of 3 M HCl to the solution in test tube 3 until the solution tests acidic with litmus paper.
17. Add 3 drops of 0.2 M K₄Fe(CN)₆ to the resulting acidic solution in test tube 3 and record observations in the data table.

Answers to Questions

1. What cation(s) are present in the unknown sample? Explain your reasoning for both the presence and absence of each ion.

Note: For best results, randomly distribute unknowns among students.

2. Write a net ionic equation for each of the following steps in the qualitative analysis scheme.

a. Separation of silver (Ag⁺) ions

Ag⁺(aq) + Cl^–(aq) → AgCl(s)

b. Confirmation of silver (Ag⁺) ions

AgCl(s) + 2NH₃(aq) → Ag(NH₃)₂⁺(aq)

c. Separation of iron(III) (Fe³⁺) ions

Fe³⁺(aq) + 3OH^–(aq) → Fe(OH)₃(s)

d. Confirmation of iron(III) (Fe³⁺) ions

Fe³⁺(aq) + SCN^–(aq) → FeSCN²⁺(aq) (red)

e. Confirmation of zinc (Zn²⁺) ions

3Zn²⁺(aq) + 2K⁺(aq) + 2Fe(CN)₆^4–(aq) → Zn₃K₂[Fe(CN)₆]₂(s)

3. Why is it necessary to use excess sodium hydroxide solution when separating iron(III) and zinc ions?

Both iron(III) and zinc hydroxide are insoluble in water. Adding excess sodium hydroxide converts Zn(OH)₂(s) to soluble Zn(OH)₄^2– complex ions. Only the iron(III) ions will precipitate out when excess sodium hydroxide is used.

4. Precipitation reactions are often used for quantitative analysis to determine the precise amount of a metal cation in solution. The most common method for the analysis of barium ions, for example, involves precipitation with sodium sulfate to form barium sulfate, which is insoluble in water.

1. Write a molecular equation and a net ionic equation for the precipitation reaction of barium chloride with sodium sulfate.

   BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
   Ba²⁺(aq) + SO₄^2–(aq) → BaSO₄(s)

2. How much sodium sulfate must be added to an aqueous solution containing 0.50 g of dissolved barium ions to ensure that all of the barium precipitates as barium sulfate?

Atomic mass of barium = 137.33 g/mole

Molar mass of Na₂SO₄ = 142.02 g/mole

5. (Optional) Review the environmental and health effects of silver ions in drinking water using the Internet. Silver ions may be used as a disinfectant to kill pathogens in water. What is the recommended maximum concentration of silver in drinking water? Are there any adverse health effects caused by silver?

The EPA recommends that the concentration of silver in drinking water should not exceed 0.1 mg per liter of water (0.1 ppm). Amounts of silver over this limit may cause blue-gray discoloration of the eyes, skin and mucous membranes. This is an unenforceable guideline (a secondary drinking water standard).

Discussion

Supplementary Information: An Example of Qualitative Analysis

Separation and Identification of Copper, Lead and Zinc Ions

Copper (Cu²⁺), lead (Pb²⁺) and zinc (Zn²⁺) ions may be naturally present in water due to erosion of rock and soil. Elevated levels of these cations may also arise due to corrosion of plumbing systems and industrial pipes. One possible qualitative analysis scheme for the separation and identification of Cu²⁺, Pb²⁺ and Zn²⁺ ions in water is shown in Figure 3.

1. Selective precipitation of lead sulfate (PbSO₄) using sulfuric acid (Equations 1a–c).

2. Precipitation of copper oxalate (CuC₂O₄) from the remaining solution using ammonium oxalate (Equations 2a and 2b).

3. The PbSO₄(s) precipitate can be redissolved in ammonium acetate (NH₄C₂H₃O₂) solution and the resulting Pb²⁺ ions converted to lead iodide, a characteristic yellow solid (Equations 3a and 3b).

4. The CuC₂O₄(s) precipitate can be redissolved with hydrochloric acid and the resulting Cu²⁺ ions converted to characteristic, royal blue complex ions Cu(NH₃)₄²⁺ by adding ammonium hydroxide (Equations 4a and 4b).

5. The remaining Zn(OH)₄^2–(aq) solution can be acidified with HCl and precipitated as Zn₃K₂[Fe(CN)₆]₂, a characteristic gray-blue solid, by adding potassium ferrocyanide [K₄Fe(CN)₆] (Equations 5a and 5b).

References

This kit was adapted from Chemical Reactions, Flinn ChemTopic™ Labs, Vol. 6, Cesa, I., Editor; Flinn Scientific Inc.: Batavia, IL (2004).

Introduction

To protect human health and safeguard the environment, the EPA regulates the amounts of many cations, including barium, copper, iron, lead, silver and zinc, in drinking water. Precise quantitative analysis of these cations is achieved using special instruments. Sometimes, however, scientists merely want to know if these cations are likely to be present in water. This question can usually be answered using qualitative analysis, in which the possible impurities in water are separated from one another and then identified using a series of chemical reactions.

Concepts

• Chemical reactions
• Qualitative analysis
• Precipitation reactions
• Complex ion reactions

Background

Both groundwater and surface water may contain significant amounts of dissolved salts (cations and anions) due to industrial discharges and agricultural runoff. Inorganic qualitative analysis of water involves carrying out a series of chemical reactions to test for the presence or absence of specific ions. In this experiment, you will design a qualitative analysis scheme for the separation and identification of three metal cations—iron(III) (Fe³⁺), silver (Ag⁺) and zinc (Zn²⁺) ions. The cations can be separated from the mixture using precipitation reactions to selectively precipitate one cation. After the cations have been separated, they can be positively identified using characteristic complex ion reactions or precipitation reactions. Consider the following relevant chemical reactions of iron(III), silver and zinc ions.

Precipitation Reactions

Fe³⁺(aq) + HCl(aq) → No Reaction
Ag⁺(aq) + HCl(aq) → AgCl(s) (white) + H⁺(aq)
Zn²⁺(aq) + HCl(aq) → No Reaction
Fe³⁺(aq) + 3NaOH(aq) → Fe(OH)₃(s) (red) + 3Na⁺(aq)
2Ag⁺(aq) + 2NaOH(aq) → Ag₂O(s) (black) + H₂O(l) + 2Na⁺(aq)
3Zn²⁺(aq) + 2K₄Fe(CN)₆(aq) → Zn₃K₂[Fe(CN)₆]₂(s) (gray-blue) + 6K⁺(aq)

Complex Ion Reactions

Zn²⁺(aq) + 4NaOH(aq) → Zn(OH)₄^2–(aq) + 4Na⁺(aq)
AgCl(s) + 2NH₃(aq) → Ag(NH₃)²⁺(aq) + Cl^–(aq)
Fe(OH)₃(s) + 3HCl(aq) + SCN^–(aq) → FeSCN²⁺(aq) (red) + 3Cl^–(aq) + 3H₂O(l)
Zn(OH)₄^2–(aq) + 4HCl(aq) → Zn²⁺(aq) + 4H₂O(l) + 4Cl^–(aq)

Experiment Overview

The purpose of this inquiry-based experiment is to design and carry out a sequence of chemical reactions for the separation and identification of iron(III) (Fe³⁺), silver (Ag⁺) and zinc (Zn²⁺) ions in water. Two parallel series of tests will be carried out, one using a known sample containing all three metal cations, the other using an unknown sample containing only one or two metals.

Materials

Prelab Questions

1. What precipitation reaction could be used to separate and remove silver ions from a mixture containing iron(III), silver and zinc ions?
2. What precipitation reaction could be used to separate and remove iron(III) ions from the remaining mixture containing iron(III) and zinc ions?
3. How can the presence of silver ions be positively identified?
4. How can the presence of iron(III) ions be positively identified?
5. How can the presence of zinc ions be positively identified?
6. Complete the following flow chart. Write the formulas of the reactants next to each numbered step and enter the formulas of the products in the boxes.

Safety Precautions

Sodium hydroxide and ammonia solutions are corrosive liquids and especially dangerous to the eyes; skin burns are possible. Ammonia is also toxic by inhalation. Hydrochloric acid is toxic by ingestion and corrosive to skin and eyes. Silver nitrate solution is a skin and eye irritant and is slightly toxic by ingestion; it will stain skin and clothing. Iron(III) nitrate solution may also be irritating to body tissues. Potassium ferrocyanide and potassium thiocyanate solutions are slightly toxic by ingestion and may liberate a toxic gas upon contact with concentrated acids. Avoid contact of all chemicals with eyes and skin. Notify the teacher and clean up all spills immediately. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the lab.

Procedure

General Techniques for Qualitative Analysis

• Keeping good records is essential for success in qualitative analysis. Number test tubes with a wax pencil or a permanent marker so the numbers will not rub off. Maintain a current, working record of observations and results—don’t trust the results to memory.
• Tap water may be a source of contamination. Rinse glassware and pipets with distilled water before use. If using a stirring rod to mix solutions, rinse the stirring rod in distilled water before transferring it to a new solution.
• In most cases, only a few drops of a concentrated precipitating agent (e.g., 3 M HCl) will be needed. Add testing solutions dropwise until no further changes in appearance are observed.
• Precipitates can be separated from the remaining liquid (called the supernatant) by centrifuging the mixture for about 30 seconds—the solid will pack down into the bottom of the test tube. After centrifuging, the supernatant can either be poured off into a second test tube or removed with a pipet. In most cases, both the precipitate and the supernatant must be carried through subsequent steps. Carefully label all test tubes to avoid confusion. Wash precipitates once with distilled water before proceeding to the next step; the rinse water may be discarded.
• Distribute test tubes symmetrically in the centrifuge to keep the rotor balanced. Never fill a test tube to capacity when placing it in the centrifuge. Leave at least 1 cm free space above the liquid level, and fill all test tubes to the same height.

1. Read the Materials section, the General Techniques for Qualitative Analysis in Lab and the recommended Safety Precautions.
2. Write a detailed, step-by-step procedure for the parallel qualitative analysis of known and unknown solutions containing iron(III), silver and zinc ions.
3. Construct a suitable data table for recording observations and results.
4. Verify the procedure and data table with your instructor and review all safety precautions.
5. Carry out the experiment and record observations in the data table. Note: “Reference” solutions of the individual metal cations will be provided to test the known reactions of each metal cation with each reagent, if desired. These optional reference tests may be helpful in deciding how much of a precipitating or complex-forming agent should be added to achieve a desired outcome.

Post-Lab Questions

1. What cation(s) are present in the unknown sample? Explain your reasoning for both the presence and absence of each ion.
2. Write a net ionic equation for each of the following steps in your qualitative analysis scheme.

1. Separation of silver (Ag⁺) ions
2. Confirmation of silver (Ag⁺) ions
3. Separation of iron(III) (Fe³⁺) ions
4. Confirmation of iron(III) (Fe³⁺) ions
5. Confirmation of zinc (Zn²⁺) ions

3. Why is it necessary to use excess sodium hydroxide solution when separating iron(III) and zinc ions?
4. Precipitation reactions are often used in quantitative analysis to determine the precise amount of a metal cation in solution. The most common method for the analysis of barium ions, for example, involves precipitation with sodium sulfate to form barium sulfate, which is insoluble in water.

1. Write a molecular equation and a net ionic equation for the precipitation reaction of barium chloride with sodium sulfate.
2. How much sodium sulfate must be added to an aqueous solution containing 0.50 g of dissolved barium ions to ensure that all of the barium precipitates as barium sulfate?

5. (Optional) Review the environmental and health effects of silver ions in drinking water using the Internet. Silver ions may be used as a disinfectant to kill pathogens in water. What is the recommended maximum concentration of silver in drinking water? Are there any adverse health effects caused by silver?